The point of a trial is to allow an independent tribunal of fact to make difficult factual decisions, like whether to believe a complainant’s oral testimony. In one of the cases cited in support of this change in practice [1] the police had access to a “computer disk containing 40,000 messages [which] revealed the alleged victim had pestered [the accused] for ‘casual sex’”, but this was not noticed until part-way through a trial which everyone had spent months preparing for. In this case, justice required that the private message history between the complainant and the accused be shared with the police and, if the case was to go to trial, the court. But the fact that the messages were discovered during the trial, rather than in the early days of the investigation, was a catastrophe for which the police apologised.
[1]: https://www.bbc.com/news/uk-england-42873618